Q: Can my mom get my iMessages from T-mobile?

No, iMessages are not carrier related.

Also check this article again:

Send messages with your iPhone, iPad, or iPod touch - Apple Support

No, however, if she has access to your Apple ID, she can see them. Perhaps it's time to have a talk with your mom about what is concerning her enough to request that from the carrier.

iMessages do not go through your cell provider. However, there are other ways that iMessages can be accessed. The really important thing is that if you are wanting to hide texts or iMessages from your parent, you may want to reconsider what you are sending. I know you probably are thinking, "Yeah, yeah, yeah, whatever," but just remember that once something is sent it is online somewhere forever. Even with apps like Snapchat that supposedly delete the information after a certain time frame, you can download other apps that can intercept the information and store it.

I don't know how old you are or what you are trying to hide from your parents. But what I can say with a high-level of confidence is that your mom loves you, wants what is best for you, and is concerned about your safety and well-being. Think about what you are downloading, viewing, or sending. Also remember that those you send texts or iMessages to may have their phones being monitored by parents and they could always contact your mom about things that you send to their child.

This may go in one ear and right out the other but just think about it.

Yes, sometimes sharing with your parents can be helpful

You're welcome